1. Field of the Invention
The present invention pertains to a method of ion chromatography wherein a specialized electrodeionization (EDI) apparatus is used.
2. Brief Description of the Background Art
This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
Ion chromatography (IC), a form of liquid chromatography, is a technique known in the art that is used for the analysis of anions and cations. IC of anions requires a different separation and detection chemistry than IC of cations.
As shown in FIG. 1, IC as typically known in the art, may include the following steps: (1) a chromatographic separation step, wherein an acid or base eluent is flowed through a chromatographic separation column to elute the ions contained within the column, (2) a suppression step, wherein the acid or base used for elution is suppressed and the ions eluted are enhanced, and (3) a detection step, wherein the eluted ions which have been enhanced are analyzed by way of an electrical conductivity detector.
In the chromatographic separation step of IC, ions of interest are typically eluted from a chromatographic separation column using an acid or base as the eluent. For the elution of anions a base is typically used and for the elution of cations an acid is typically used. It is desirable to use a relatively pure acid or base as the eluent, so that contaminant anions and cations are not introduced to the eluent. Thus, there is a need in the art for a convenient source of high purity acid and high purity base for use as an eluent in IC.
In the ion suppression step of IC, electrical conductivity of the acid or base eluent is suppressed, while the conductivity of the separated ions of interest are enhanced to aid in their detection. As is typical in the art for IC of anions, the base eluent will be suppressed (neutralized) in an anion suppressor and for IC of cations, the acid eluent will be suppressed (neutralized) in a cation suppressor. Techniques known in the art for ion suppression are described in detail in U.S. Pat. No. 3,926,559, which is hereby incorporated by reference.
The history of ion chromatography suppression as of 1993 was summarized in Rabin, S. et al., New Membrane-Based Electrolytic Suppressor Device for Suppressed Conductivity Detection in Ion Chromatography, J. of Chromatog. 640 (1993) 97-109, also incorporated herein by reference.
Many ion suppression methods described in the background art, for example U.S. Pat. No. 3,926,559, used a bed of ion exchange resin particles commonly referred to as a packed bed suppressor (PBS). The PBS requires periodic regeneration by flushing with an acid or base solution. While packed bed suppressors have proven useful in ion chromatography, there are a number of disadvantages of a PBS. These disadvantages include a) periodic regeneration of the PBS which interrupts sample analysis; b) a loss of resolution due to band broadening in the PBS over time; and c) changes in retention of certain analytes as a function of the degree of exhaustion of the PBS. To alleviate this problem, continuously regenerating electrolytic ion suppressors were introduced to the art. A few examples of these suppressors are listed below.
U.S. Pat. Nos. 5,248,426 and 5,352,360 to Stillian et al., issued Sep. 28, 1993 and Oct. 4, 1994, respectively describe a method and apparatus for a form of suppressor that uses a direct current power controller, which generates an electric field across two platinum electrodes to electrolyze water in regenerant channels of the apparatus. Functionalized ion-exchange screens are present in the regenerant chambers to facilitate electric current passage with the permselective ion-exchange membrane defining the chromatography eluent chamber.
U.S. Pat. Nos. 6,077,434 and 6,328,885 to Srinivasan et al., issued Jun. 20, 2000 and Dec. 11, 2001, respectively, describe a method and apparatus for increasing the current efficiency of suppressor and suppress-like pretreatment devices for the purpose of suppressing a high concentration of eluent.
U.S. Pat. Nos. 6,325,976; 6,495,371; and 6,508,985 to Small et al., issued Dec. 4, 2001, Dec. 17, 2002, and Jan. 21, 2003 respectively, describe an electrolytic suppressor, which includes a suppressor bed of ion exchange resin, an electrode chamber with electrodes in contact with the resin bed, and a recycle conduit between the suppressor outlet port and the electrode chamber, and a method for using such electrolytic suppressor.
U.S. Pat. No. 6,562,628 to Liu et al., issued May 13, 2003 describes a combination electrolytic suppressor and separate eluent generator and method. The suppressor includes a chromatography effluent flow channel, an ion receiving flow channel, and a first suppressor ion exchange barrier there between permeable to electrolyte ions but not liquid flow. Acid or base electrolytically generated in the first generator electrode chamber flows as an eluent stream to the chromatographic separator.
While electrolytic suppressors are the predominant suppressor in IC, there are draw backs to this type of suppressor. The eluent anion for use in cation analysis is drawn towards the anode. The oxidative nature of the anode electrochemistry limits the choice of eluents to non-electrochemically active anions such as sulfate or methane sulfonate. Common mineral acids such as hydrochloric acid or nitric acid cannot be used because at the anode, they are oxidized and the reaction products (such as hypochlorite) damage the anion exchange material. This results in increased resistance of the electrolytic suppressor, poor suppression capacity and ultimately failure of the suppressor.
In cation analysis, where a sample may contain large quantities of chloride, such as brine or seawater, the chloride from the sample is also removed from the suppressor and can cause a similar type of damage. Thus, there is a need in the art for a cation suppressor compatible with chloride, nitrate and other electrochemically active anions.